Credit Note for 19-20 and 21-22 Invoice - Goods and Services Tax - GST

I have a Custiomer who is not paying for my Invoice of 19-20 and 21-22. Can I raise a Credit note now and claim the ITC next month GSTR-3B ? 19-20 there was no GSTR-9C requirement, 21-22 GSTR filed on Dec31-2022. Can we modify GSTR-9C once filed? Thanks and regards Pankaj Gupta